Fuel-borne organometallic combustion catalysts are becoming important in the control of environmental pollutants where fossil fuel is burned, for example, in internal combustion engines and in stationary burners such as those used in home heating, industrial furnaces, and steam power generators that burn fuel oil or coal. Organometallics derived from transition metals such as manganese, cerium, platinum, iron, and molybdenum; alkali metals such as lithium, sodium, and potassium; and alkaline earth metals such as calcium, magnesium, strontium and barium can all serve variously as effective fuel-borne emission control catalysts for controlling production of soot, smoke, hydrocarbon, carbon monoxide, sulfur trioxides, and oxides of nitrogen emissions. However, the vast majority of fossil fuels contain certain contaminant elements such as sulfur, phosphorus, vanadium, etc. that bind with these fuel-borne catalysts and inhibit them from efficiently performing their intended purpose. In addition, some combustion units are constructed with metals, such as iron, which can oxidize, corrode, or poison an emission control system.
High-temperature corrosion (above 400° C.) occurring on hot surfaces of the combustion unit is promoted by fuel contaminants such as sodium, vanadium and iron. This corrosion is promoted by oxygen near and on the surfaces in question. Sodium vanadate combustion products absorb this oxygen to form low-melting sodium vanadylvanadate fluxes that oxidatively corrode and physically erode the metal surface by forming a corrosive and free-flowing surface alloy with the metal. Fuels such as coal that have high levels of iron give an ash surface deposit whose fusion temperature falls with iron concentration increase in the fuel. The iron in effect lowers the fusion temperature of the slag, and just like the low melting sodium vanadylvanadate fluxes described above, this molten slag corrodes and erodes the metal surface in a similar manner.
Low-temperature corrosion (below 250° C.) can occur in the cooler region of a combustion unit towards the exhaust stack. Fuel sulfur, sodium, vanadium and iron can cause this corrosion. Sodium vanadylvanadates and iron, in the presence of oxygen, both independently catalyze conversion of SO2 to SO3 at high temperatures and the resulting SO3 hydrates with combustion water at lower temperatures in the exhaust stack to give corrosive sulfuric acid. The key to both high- and low-temperature corrosion processes is availability of oxygen near species and surfaces capable of shunting it into the corroding surfaces. To avoid an expensive maintenance cost, corrosion must be inhibited or slowed down. It would therefore be desirable to scavenge the oxygen used to promote the corrosion.
Fuel-borne organometallic combustion and emissions control catalysts, such as those of cerium, platinum, manganese, and iron are used in engines and burners both to lower exhaust particulates, NOx, hydrocarbon, etc, and as light-off catalysts for passive diesel particulate filters (DPFs), catalyzed diesel particulate filters (C-DPFs), and continuously regenerating technology diesel particulate filters (CRT-DPFs) used to filter particulate from the exhaust stream. The true efficiency of these catalytic additives is compromised by the presence in the fuel, or from the surfaces of the combustion unit, of certain metal contaminants with which these additives have to react first. In doing so, a portion of the catalytic activity of the additive is sacrificed and is no longer available to perform its intended task.